Separation
In air traffic control, the risk of aircraft colliding is managed by applying separation rules. These rules require aircraft to be separated by either a minimum vertical distance, or if vertical separation is not feasible, by a minimum horizontal distance defined by various means. One of the means of determining horizontal separation is by a controller observing the radar returns of the aircraft to be at least a minimum horizontal distance apart. This is the essence of radar control and is probably the form of air traffic control most familiar to lay people.
However in times gone by radar was not very common, and in certain parts of the world today it still is not common, on grounds of cost or technical feasibility. Procedural control is a form of air traffic control that can be provided to aircraft in regions without radar, by providing horizontal separation based upon time, the geography of predetermined routes, or aircraft position reports based upon ground-based navigation aids, for those aircraft that are not vertically separated.
Read more about this topic: Procedural Control
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